Hi Laura!
There is a huge list of legitimate mystery shopping companies linked at the bottom of this page. Start by signing up with the "15 Most Discussed" as they are all larger companies with work nationwide.
In terms of "finding shops that pay," what, specifically are you looking for and how much time do you have to spend mystery shopping? How you approach making money will very much depend on what type of time you have to commit to it. I work full time, but don't have young kids, so I spend quit a bit of time outside my full time work mystery shopping, but can easily pull in over $2000 in any month I want.
Hard work builds character and homework is good for your soul.